1. Field of the Invention
The present invention relates to a method for producing a high purity ultra fine silica powder.
2. Discussion of Background
Heretofore, an ultra fine silica powder having a large specific surface area has been packed for e.g. rubbers and resins for reinforcement thereof. The ultra fine silica powder may, for example, be silica obtained by flame thermal decomposition of silicon tetrachloride (fumed silica) or precipitated silica made of sodium silicate solution (precipitated silica), and it has an extremely large specific surface area of a level of from 50 to 500 m.sup.2 /g.
However, the fumed silica is high-priced since an expensive silicon tetrachloride gas is employed as a raw material, and the precipitated silica has a low purity of a level of 95%, and its use is limited, although it is available at a relatively low cost. Further, these methods are not always adequate as a mass production process, and it has thereby been desired to develop a mass production process to produce a high purity ultra fine silica powder at a low cost.
Accordingly, as a production method by using a silica raw material of low price, a method of spraying a slurry of a silica raw material and an inflammable liquid into a flame (JP-A-10-297915) or a method of quenching vapor formed by heating a silica raw material at a temperature of at least the boiling point of the silica (JP-A-2-233515) may, for example, be proposed.
However, the technique as disclosed in JP-A-10-297915 is basically a melting process by a high temperature flame, and the average particle size of the obtained silica particles is a level of several .mu.m, and does not reach the ultra fine powder level of submicron. On the other hand, since the method as disclosed in JP-A-2-233515, comprises a vapor phase reaction, ultra fine powder can be obtained. However, heat treatment at a temperature of at least the boiling point of the silica (2230.degree. C.) is required, and accordingly, the product will be contaminated by impurities in the furnace material and will have a low purity, or it may be necessary to employ an extremely expensive furnace material improper for an industrial process.